Heat Treating?
It wouldn't stop, but it takes time and if you are a medieval bowyer making them for war you wouldn't have time.
I saw a recent document for 4 dozen Yew bows made and delivered in a month and that was for the US government in 1942
that's more than 1 per day.
(source: Billets to Bows by Glen St Charles)
If that is the turnaround in 1940 imagine the workload in the run up to time of Agincourt... I'd suggest there was little time for heat treating.
(That's not to say a bowyer might not do it for some special bow)
Del